In order to avoid loss of data, SoSci Survey does not allow deleton of data while the surve is online. I strongly recommend not to bypass this security feature by removing the survey administration period, and then setting it after deleting the files.
Instead please send an email to email@example.com including (a) an estimate how much storage space will be required overall, (b) the URL of the respective survey project, and (c) a confirmation which files you have downloaded and (!) have created a backup from on an external drive.
Depending on ressource required and availability we check if the storage can be increased for the survey project. If not, we can delete the files.